Recently, there has been increasing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. Under these circumstances, rechargeable secondary batteries in particular have attracted considerable attention as the most promising electrochemical devices. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Many secondary batteries are currently available. Lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries such as Ni—MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire or explosion, encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient discharge capacities at low temperature of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers, such as fire and smoke emission, of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher on account of their material properties and in view of manufacturing processing including elongation. This thermal shrinkage behavior may cause short-circuiting between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, a separator has been suggested in which a mixture of excessive inorganic particles and a binder polymer is coated on at least one surface of a highly porous substrate to form a porous coating layer.
The presence of a sufficient amount of the inorganic particles above a predetermined level is a prerequisite for the above-mentioned advantageous functions of the organic-inorganic composite porous coating layer formed on the porous substrate. However, an increase in the content of the inorganic particles, i.e. a decrease in the content of the binder polymer, reduces the bindability of the separator to the electrodes and causes separation of the inorganic particles from the porous coating layer when stress occurs during assembly (e.g., winding) of the electrochemical device or the separator contacts external members. The separated inorganic particles act as local defects of the electrochemical device, giving a negative influence on the safety of the electrochemical device.
Thus, there is a need to develop a method for manufacturing a separator having high bindability and improved ability to prevent separation of inorganic particles.